Vincent Hardy is a Java Architect at Sun's Java Solution Center in Menlo Park, CA and the author of the upcoming book: Java 2D Graphics. He can be contacted at [email protected].

How can we use transforms in animation? Let's look at a simple object trajectory, as in Figure 1.


Figure 1. A simple object trajectory.

The object here is represented by a small triangle and its successive positions along the path are numbered from 1, the initial position, to 5, the final position. In this example, each point on the path can be seen as a transform of the origin. To create an animation that paints a Shape object at the positions 1 through 5, we can first create the object centered about the origin and then, each time a new frame is rendered (a frame is one of the successive images in the animation), we can simply modify the graphics transform to an increasingly big translation, as shown by the blue arrows in Figure 2. The Shape will appear to move along a straight line path. The same use of transforms can be applied for more sophisticated trajectories, but also other effects. For example, applying increasingly large rotations for each frame creates the illusion that an object is spinning.


Figure 2. Modifying the graphics transform.

Our example uses that idea: A Shape object is transformed by several sequences of transforms to make it roll, fly, and spin.

Before we discuss different animation effects, let's have a look at how the animation itself is done. Our top-level class (Listing 10) is called CompositionDemo and is a javax.swing.JComponent. It takes the string of text that should be animated as an input parameter and, as we will see, starts the animation on a mouse button click (see Listing 1).

In its constructor, our class prepares the Shape object (see Listing 2) it will render and the Paint objects it will use to render it (see the updateBuffer method we describe later).

DOUBLE BUFFERING
We use the double buffering technique to avoid flickering when rendering to the screen. First we create an offscreen buffer:

BufferedImage buffer;
   . . .
  buffer = new BufferedImage(w, h,
BufferedImage.TYPE_INT_RGB);  // Creates an RGB image

  g = buffer.createGraphics();

  public void paint(Graphics _g){
    _g.drawImage(buffer, 0, 0, this);
  }

  
public boolean isDoubleBuffered(){
    return true;
  }

  addMouseListener(new MouseAdapter(){
      public void mouseClicked(MouseEvent evt){
         System.out.println("Starting animation");
         startAnim();
      }
    });

  public void startAnim(){
    if(animThread!=null && animThread.isAlive()){
      // If a thread is already started try
      // to stop it gracefully
      animThread.pleaseDie();
      animThread = null;
    } 
    animThread = new AnimThread();
    animThread.start();
  }

interface TransformGenerator {
  /** @return the next transform */
  public AffineTransform next();

  /** @return whether or not there are more transforms */
  public boolean hasNext();

  /** Resets the generator so that it starts from the 
		   first transform again */
  public void reset();

  /** @return total number of transforms generated */
  public int getTransformCount();
}

 /** 
Set of transform generators */
  TransformGenerator generators[];
  ...
  generators = new TransformGenerator[] {
      new RollingGenerator(
       r.width/2, h/2, w-r.width, 3, 40),
      new TranslationGenerator(
       w-r.width/2, h - r.height/2, -w+r.width, 
       -h+r.height, 15),
      new SpinningGenerator(w/2, h/2, 1, 4.f, 15)
      };